The interaction of leukocytes with venular endothelial cells (EC) represents a key control point in leukocyte recruitment from the blood, and thus is central to the regulation and maintenance of local immune and inflammatory reactions. Leukocyte-EC interactions are regulated by receptors that mediate sequential attachment, rolling, leukocyte activation, activation-dependent arrest, and diapedesis. This proposal targets the physiology and molecular biology of a newly identified trafficking receptor, L11, involved in monocyte and T cell homing during immune and inflammatory responses. L11 is defined by a monoclonal antibody (MAb) that blocks monocytoid cell binding to inflamed high endothelial venules (HEV). The antigen is expressed by T cells (but not B cells) as well as monocytes, and is distinct from known adhesion or activating receptors. Under this proposal, L11 will be characterized biochemically, and its cellular distribution and regulation will be assessed, focusing in particular on its regulation during lymphocyte and monocyte differentiation, development, and activation. The involvement of L11 in monocyte interactions with inflamed EC and in monocyte trafficking will be explored by assessing the effects of anti-L11 MAB on in vitro adhesion of monocytoid cells to inflamed or cytokine-activated EC; and on monocyte recruitment to the experimentally inflamed peritoneum in vivo. Participation of L11 in T cell-EC interactions and T cell trafficking will be confirmed in parallel in vitro assays, and in conventional in vivo studies of lymphocyte homing. In situ videomicroscopic analyses of physiologic interactions with HEV, and with inflamed venules, will be carried out to define the level at which L11 participates in the multi- step process of EC recognition and extravasation in vivo: (i.e., does anti-L11 inhibit attachment, rolling, leukocyte activation, arrest, or diapedesis). Results will be compared with ongoing studies of the involvement of selectins and integrins in these in situ events. cDNA's encoding L11 will be isolated by expression cloning. Homologies to known receptor families may provide further insight into L11 function(s), and may direct subsequent structure/function analyses. Stable lymphoid cell or CHOP transfectants and, if indicated, soluble L11-immunoglobulin constructs will be used as probes of L11 function (does L11 bind to HEV or to inflamed endothelium? does its expression on transfectants confer enhanced adhesion?). Finally, the potential of L11 as a therapeutic target will be explored by asking if L11 MAb can inhibit or suppress pathologic inflammation in mouse models of DTH or of autoimmune and inflammatory diseases. Physiologic and molecular genetic characterization of this novel trafficking receptor may open up new possibilities for therapeutic modulation of pathologic immune responses.